I. Field of the Invention
The present invention relates to a photoconductive target of a vidicon type image pickup tube and, more particularly, to an improvement in a photoconductive target which has a high transference sensitivity in a visible light range and a good after image characteristic.
II. Description of the Prior Art
As materials of a photoconductive target of a vidicon type image pickup tube, Sb.sub.2 S.sub.3, PbO, Si, Se-Te-As, ZnSe-ZnCdTe, CdSe-CdSeO.sub.3 -As.sub.2 Se3 and the like are known. Among these materials, a photoconductive target containing PbO or Se-Te-As as a major component has a relatively low sensitivity but a low dark current and short response time. For this reason, such a photoconductive target is used for forming color images. Although a target using a ZnSe-ZnCdTe or CdSe material has a quantum efficiency which is substantially 1 unity and has a poor after image characteristic, i.e., relatively long response time as compared to the PbO target described above, it has a high sensitivity about 10 times that of the PbO target.
The present inventors have proposed a photoconductive target having a low dark current characteristic and a high sensitivity not only in a visible light region but also in an infrared light region. In this photoconductive target, a photoconductive layer is formed of a material represented by the general formula CdTe.sub.(1-x) Se.sub.(x) (where (x) is 0.3 to 0.5) so as to be considerably thick, e.g., 1.5 .mu.m. A target layer comprising a high resistance As.sub.2 Se3-system amorphous semiconductor layer formed on the photoconductive layer to be relatively thin, e.g., 1.5 .mu.m. Although the target has high sensitivity up to an infrared light region as described above, the after image characteristic is not as good as that obtained with the PbO film.